1. Field of the Invention
The present invention relates to a high-density magnetic recording medium excellent in both of electromagnetic characteristics and running durability.
2. Description of Related Art
Magnetic recording media have been widely used as tapes for recording tapes, video tapes, floppy discs and the like. Recently, it is generally required to provide magnetic recording media having especially excellent electromagnetic characteristics. For example, magnetic recording media used as audio tapes for music recording and reproducing have been required advanced reproduction ability of fundamental tones, while magnetic recording media used as video tapes have been required advanced reproduction ability of original pictures.
In addition to such excellent electromagnetic characteristics, magnetic recording media have been also required to exhibit good running performance and running durability. To improve the running durability, abrasives and lubricants are generally added to magnetic layers. In order to materialize excellent running durability, it is required to increase their added amounts to some extent. However, if the added amounts are increased, the filling density of a ferromagnetic powder is necessarily lowered. Especially, if the added amount of lubricant is increased, the binder is easily plasticized, the durability of the magnetic layer inversely tends to be lowered. In addition, if an abrasive having large particle diameter is used in order to materialize excellent durability, the abrasive is easily projected excessively on the surface of the magnetic layer. All of these lead to lowering of the electromagnetic characteristics. In addition, such methods have been developed as to select a certain binder and add the same. However, all of those methods have not been attempted to solve the problems, such as lowering of dispersibility and deteriorating of running property (Japanese Unexamined Patent Publication (KOKAI) Showa No.61-148,626, Japanese Unexamined Patent Publication (KOKAI) Showa No.61-190,717, Japanese Unexamined Patent Publication (KOKAI) Showa No.62-82,510, Japanese Unexamined Patent Publication (KOKAI) Heisei No.1-251,416, Japanese Unexamined Patent Publication (KOKAI) Heisei No.1-267,829, Japanese Unexamined Patent Publication (KOKAI) Heisei No.3-44,819, Japanese Examined Patent Publication (KOKOKU) Heisei No.6-64,726, Japanese Unexamined Patent Publication (KOKAI) Heisei No.6-96,437, Japanese Examined Patent Publication (KOKOKU) Heisei No.7-21,851).
Therefore, such attempts have been developed in order to improve the running durability as to provide base projections not on a magnetic layer but on a backcoat layer, and to make surface rough by addition of coarse carbon particles having a particle diameter equal to or higher than 0.1 xcexcm. However, if the surface of the backcoat layer is roughened by such methods, when the magnetic recording medium wound up is preserved and processed, the backcoat layer contacts with the magnetic layer under pressure, thereby transfer of unevenness of the backcoat layer to the magnetic layer, so-called, xe2x80x9creverse transfer xe2x80x9d occurs. As a result, the electromagnetic characteristics may disadvantageously be lowered.
To solve such a problem of xe2x80x9creverse transferxe2x80x9d, such an attempt has been developed as to make the surface of a backcoat layer smooth. However, if the surface of the backcoat layer is smoothened, friction coefficient to guides of a reproduction machine such as a VTR increases, thereby the running stability becomes lowered. Therefore, in order to lower the friction coefficient, such an attempt has been developed as to add a relatively large amount of a lubricant to a backcoat layer. However, if a large amount of a lubricant is used, another problem causes that the guides stain by increased adherence of the surface of the backcoat layer. As described, in order to prepare magnetic recording media having excellent electromagnetic characteristics and good running stability, further improvements to a backcoat layer are required.
On the other hand, recently, magnetic recording media are required to record signals in still more high-density. Therefore, recently, it tends to make the magnetic layer thinner in order to improve outputs. For example, as for particulate magnetic recording media, such a technique has been practically used that a nonmagnetic layer and a magnetic layer are coated in this order on a flexible support by simultaneous multilayer coating, and thereby the magnetic layer as an upper layer is made thinner (Japanese Examined Patent Publication (KOKOKU) Heisei No.4-71,244). In addition, also in vapor deposited (ME) tapes, the output is made higher by making the thickness of the magnetic layer within the range of from 0.15 to 0.2 xcexcm.
It is required to reduce the thickness of a layer not only for a magnetic layer but also for each layer constituting magnetic recording media. Because recent media for computer storage, especially, tapes for small backup systems typified by D8 and DDS are required to have higher capacity, it is necessary to make the whole thickness of the medium thinner. In order to make the thickness of magnetic recording medium thinner, it has been conventionally attempted to make a thinner flexible support and a nonmagnetic layer provided between a flexible support and a magnetic layer of a particulate magnetic recording medium. However, if a flexible support was made thinner as having a thickness below a certain range, running durability was lowered. If a nonmagnetic layer was made thinner, output lowered, error rate increased and dropout increased.
A backcoat layer cannot be omitted for keeping good running property in small backup systems such as D8 and DDS adopting helical scan recording systems. In addition, if a backcoat layer is made thinner, the backcoat layer is peeled off in repeated running due to lowering the adhesion strength to the flexible support,or projections on the surface of the backcoat layer transfer to the magnetic layer, thereby the output becomes lowered. Especially, for magnetic recording media for DDS-2 and DDS-3, an aramid base is used as a flexible support in order to increase electromagnetic characteristics and running durability. However, falling and scraping of the backcoat layer at the end surface of the edges in repeated running are remarkable compared with a polyester base conventionally used, such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). Moreover, by increasing the Young""s modulus in a width direction of a flexible support, the strength in a width direction of a thin magnetic recording medium is kept, so that the running durability is secured. However, it has such a problem that parts of the end surface of the backcoat layer (that is, the end surface formed by slitting) are fallen when the magnetic layer is prepared by slitting.
As described above, the magnetic recording medium satisfying both of electromagnetic characteristics and running durability could not be provided in prior art. Especially, in today when the whole thickness of the magnetic recording medium is required to be thinner, further improvements are required for enhancing electromagnetic characteristics and running durability.
Therefore, it is an object of the present invention to solve the problems of the prior art, and to provide a magnetic recording medium exhibiting good running durability, high output and high capacity.
The present inventors have diligently studied to solve aforementioned problems, and as a result, they found that a magnetic recording medium having good running durability and excellent electromagnetic characteristics can be provided by incorporating titanium oxide or xcex1-iron oxide, as an inorganic powder into a backcoat layer, in an amount larger than that of carbon black and decreasing the amount of a binder.
That is, the present invention provides a magnetic recording medium comprising a magnetic recording medium on a surface of a flexible support, and a backcoat layer on the opposite surface, characterized in that the backcoat layer contains a granulated oxide consisting of titanium oxide, xcex1-iron oxide or a mixture thereof (in this specification, xe2x80x9cgranulated oxidexe2x80x9d entirely means titanium oxide, xcex1-iron oxide or a mixture thereof) and carbon black in a weight ratio of from 60/40 to 90/10, further contains a binder in an amount of from 10 to 40 parts by weight with respect to 100 parts by weight of the total weight of the granulated oxide and the carbon black.
As the granulated oxide for the backcoat layer of the magnetic recording medium of the present invention, for example, a spherical oxide having a particle diameter of from 0.01 to 0.2 xcexcm, and an acicular oxide having a length of the major axis of from 0.05 to 0.3 xcexcm and a length of the minor axis of from 0.01 to 0.05 xcexcm can be used. In addition, as carbon black for the backcoat layer, for example, a carbon black having a particle diameter of from 0.01 to 0.3 xcexcm can be used.
In addition, the backcoat layer preferably contains a polyurethane resin as a binder. Particularly preferable resins are reaction products of a diol and an organic diisocyanate and the above diol consists of those which contain a short chain diol having a cyclic structure in an amount of from 17 to 40% by weight and a long chain diol having an ether linkage in an amount of from 10 to 50% by weight, respectively, with respect to the polyurethane resin wherein the above ether linkage in the long chain diol is contained in an amount of from 1.0 to 5.0 mol/g with respect to the polyurethane resin.
The thickness of the whole magnetic recording medium preferably ranges from 3 to 7.5 xcexcm.
The flexible support of the present invention preferably has the surface roughness (Ra) on the backcoat layer side equal to or less than 4 nm. In addition, it is preferable that the flexible support is an aramid base and the adhesion strength between the backcoat layer and the flexible support is equal to or higher than 80 g/(8 mm width). A nonmagnetic layer comprising a nonmagnetic inorganic powder and a binder may be provided between the flexible support and the magnetic layer.